This invention relates generally to investment casting and, more particularly, to a method of increasing the strength and solids level of investment casting shells.
Investment casting, which has also been called lost wax, lost pattern and precision casting, is used to produce high quality metal articles that meet relatively close dimensional tolerances. Typically, an investment casting is made by first constructing a thin-walled ceramic mold, known as an investment casting shell, into which a molten metal can be introduced.
Shells are usually constructed by first making a facsimile or pattern from a meltable substrate of the metal object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene or plastic.
Next, a ceramic shell is formed around the pattern. This may be accomplished by dipping the pattern into a slurry containing a mixture of liquid refractory binders such as colloidal silica or ethyl silicate, plus a refractory powder such as quartz, fused silica, zircon, alumina or aluminosilicate and then sieving dry refractory grains onto the freshly dipped pattern. The most commonly used dry refractory grains include quartz, fused silica, zircon, alumina and aluminosilicate.
The steps of dipping the pattern into a refractory slurry and then sieving onto the freshly dipped pattern dry refractory grains may be repeated until the desired thickness of the shell is obtained. However, it is preferable if each coat of slurry and refractory grains is air-dried before subsequent coats are applied.
The shells are built up to a thickness in the range of about xe2x85x9 to about xc2xd of an inch (from about 0.31 to about 1.27 cm). After the final dipping and sieving, the shell is thoroughly air-dried. The shells made by this procedure have been called xe2x80x9cstuccoedxe2x80x9d shells because of the texture of the shell""s surface.
The shell is then heated to at least the melting point of the meltable substrate. In this step, the pattern is melted away leaving only the shell and any residual meltable substrate. The shell is then heated to a temperature high enough to vaporize any residual meltable substrate from the shell. Usually before the shell has cooled from this high temperature heating, the shell is filled with molten metal. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the casting mold has solidified and cooled sufficiently, the casting may be removed from the shell.
Although investment casting has been known and used for thousands of years, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment casting shells more efficiently, cost-effective and defect-free. For instance, if the strength of investment casting shells could be increased, less material would be required. The stronger shells would also be more crack resistant, thereby resulting in castings with fewer defects. Furthermore, if the solids level of investment casting shells could be increased, the shells would dry faster and be made with fewer coats for additional time, material and cost savings.
Accordingly, it would be desirable to provide an improved method of increasing the strength and solids level of investment casting shells.
The method of the invention calls for incorporating at least one microsilica into an investment casting shell. The addition of the microsilica effectively increases the strength and solids level of the investment casting shell.
The present invention is directed to a method of increasing the strength and solids level of investment casting shells. In accordance with the invention, at least one microsilica is incorporated into the shell. The microsilica can be introduced into the investment casting shell by adding the microsilica to the slurry via any conventional method generally known to those skilled in the art.
The microsilicas which may be used in the practice of the invention include man-made microsilicas such as silica fume and fumed silica, naturally-occurring microsilicas, known as pozzolans, and mixtures thereof. Examples of suitable pozzolans include diatomaceous earth, opaline cherts and shales, tuffs, volcanic ashes, pumicites and fly ash. The preferred microsilica for use in increasing the strength and solids level of investment casting shells is silica fume. By definition, xe2x80x9csilica fumexe2x80x9d is a by-product of silicon, ferrosilicon or fused silica manufacture.
The microsilica is used at a concentration which will effectively increase the strength and solids level of an investment casting shell. It is preferred that the amount of microsilica which is added to the shell be in the range of about 0.1 to about 15.0% by weight of the shell. More preferably, the amount of microsilica is from about 0.2 to about 10.0%, with about 0.5 to about 5.0% being most preferred.
The present inventor has discovered that incorporating at least one microsilica into an investment casting shell effectively increases the strength and solids level of the shell. The inventor has also found that microsilica additions create stronger shells with fewer coats, thus providing for material savings and productivity enhancement, as well as higher quality molds to produce castings with fewer defects.